This invention relates to the field of motor speed control systems, particularly to systems that control the motor of a color wheel in a display system.
Many projection display systems use a single light modulator in combination with a white light source to produce a full color image. In order to produce a full color image, the white light source is filtered sequentially to produce a primary colored light beam that changes over time. Typically, a color wheel is used to allow a series of primary colored filters to be spun through the white light beam in rapid succession. As each filter passes through the light beam, the light beam becomes a primary color beam with the active primary color determined by the which portion of the color wheel is passing through the optical path.
During each primary color period, data for the appropriate color is provided to a spatial light modulator to enable the modulator to create a series of single color images. If the single color images are created in a rapid sequence, the viewer""s eye integrates the series of images to give the perception of viewing a single full-color image.
Because the data that must be written to the modulator depends on the position of the color wheel, the position of the color wheel is tightly controlled to synchronize the color wheel with the remainder of the display system. The transition period between adjacent color filtersxe2x80x94typically called a spoke periodxe2x80x94requires turning the modulator off to ensure only pure primary colored light is used to create each of the three primary colored image. Uncertainties and errors in the position or speed of the color wheel force the display system controller to lengthen the spoke periods to ensure only primary colored light is incident the modulator at the appropriate time. Unfortunately, the accumulated off time associated with the lengthened spoke periods creates a substantial drop in projector efficiency.
An accurate and efficient method and system for controlling the color wheel is needed so that the duration of each spoke period can be kept to a minimum and the efficiency of the display system will be increased.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention that provides a method and system for increasing the resolution of a motor control circuit. According to one embodiment of the disclosed invention, a motor controller for controlling the speed of a motor is provided. The motor controller is comprised of a motor control circuit electrically connected to the motor for receiving a digital speed control word representing a desired motor speed and driving the motor, a controller connected to the motor control circuit for providing the digital speed control word to the motor control circuit, the controller dithering the digital speed control word at least twice each revolution of the motor.
Another embodiment of the disclosed invention provides a display system comprising a light source for producing a beam of white light along a first light path, a filter wheel on the first light path for filtering the beam of white light, a motor connected to the filter wheel for spinning the filter wheel, a motor control circuit electrically connected to the motor for receiving a digital speed control word and driving the motor, a spatial light modulator on the first light path for receiving and selectively modulating the filtered beam of light traveling along the first path to form an image, and a controller for providing image data to the spatial light modulator and the digital speed control word to the motor control circuit. The digital speed control word is dithered at least twice each revolution of the motor. The digital speed control word typically is dithered each time the controller receives a commutation interrupt form the motor control circuit.
The disclosed invention provides the technical advantage of increasing the effective resolution of the motor controller speed command word. This in turn increases the accuracy of the control loop as well as the frequency and phase stability of the motor control loop. The increased resolution is obtained at no additional hardware cost through relatively simple software control and uses minimal microcontroller resources.